1. Field of the Invention
The present invention relates to a voltage generator-booster for supplying a pulsating voltage having approximately constant voltage level.
2. Discussion of the Related Art
Integrated circuits currently demand voltage levels that remain substantially constant despite a variation in supply voltage. The voltage levels may be higher than the supply voltage, in which case a voltage generator (booster) for increasing the supply voltage is called for, or the voltage levels may be lower, in which case the voltage generator provides for limiting the supply voltage. Generation of the high voltage is important, for example, in the case of nonvolatile memories with a 3 V supply voltage, the cells of which must therefore be read at a higher gate voltage; whereas, in the presence of a high supply voltage, this must be limited to prevent the reading of programmed cells.
If a circuit capable of operating at both high and low supply voltages is required, with a voltage generator for supplying an approximately constant voltage level despite a variation in supply voltage, the generator must be capable of operating as both a voltage booster and limiter, depending on the supply voltage.
At present, known voltage boosters include a charge pump circuit which, in turn, includes a bootstrap capacitor and a pull-up circuit which operates in two states. The first state precharges the capacitor to roughly the supply voltage, where a first terminal of the capacitor is grounded and a second terminal is connected to the supply voltage. In the second state, the first terminal of the capacitor is pulled up to the supply voltage, and the second terminal (which is therefore at nearly twice the supply voltage) is connected to an output. Even though the actual output voltage depends on the charge distribution between the bootstrap capacitor and the capacitance of the load supplied by the booster, so that the final output voltage is less than twice the supply voltage, the output voltage of known boosters is highly dependent on supply voltage, thus preventing the known boosters from being employed in devices operating within a wide range of different supply voltages, e.g., 3 to 7 V.